Hitherto, in development systems employing nonmagnetic monocomponent developer, a contact development method in which an elastic roller is used is widely employed, as described in Japanese Patent Laid-Open No. 2005-164756. A process cartridge mountable into and demountable from an image forming apparatus includes a drum unit and a development unit. The drum unit includes a photosensitive drum on which an electrostatic latent image is to be formed, a charging roller, and a cleaning device. The development unit includes a developing roller and a toner container.
In the contact development method, the developing roller having a layer of developer formed on the surface thereof is brought into contact with the photosensitive drum having an electrostatic latent image formed thereon. Subsequently, an electric field is produced between the photosensitive drum and the developing roller, whereby the electrostatic latent image on the photosensitive drum is visualized with the developer.
To bring the developing roller into contact with the photosensitive drum, the drum unit and the development unit are rotatably connected to each other at a connecting portion and are pressed against each other with a spring. Furthermore, to transmit a driving force to the developing roller, a driving gear having the center of rotation thereof at the connecting portion is supported and transmits the driving force from a drive motor to the development unit.
Meanwhile, as disclosed in Japanese Patent Laid-Open No. 2001-66970, another configuration is proposed in which a development unit is connected to a drum unit in such a manner as to be movable in the substantially horizontal direction in a state where a process cartridge is mounted in an image forming apparatus.
In the contact development method, however, the contact pressure produced between the photosensitive drum and the developing roller needs to be controlled. A contact pressure over a specific range of values causes problems such as deterioration of developer due to an overload on the developer provided at the point of contact and fusion of developer onto the developing roller. In contrast, a contact pressure below the specific range of values causes problems such as image dropouts and a reduction in the image density due to a reduced development efficiency. To avoid such problems, in the process cartridge described in Japanese Patent Laid-Open No. 2005-164756, the contact pressure is determined on the basis of the urging force of the spring and the driving force of the driving gear. The driving force changes with changes in internal loads of the development unit, such as a change in a developer-stirring load and a change in the frictional force at the point of slidable contact occurring in the development unit. Along with such changes, the contact pressure may also change. On the other hand, in the process cartridge disclosed in Japanese Patent Laid-Open No. 2001-66970, the contact pressure produced between the photosensitive drum and the developing roller is maintained to be constant, without being affected by the weight of the development unit. However, since the development unit slides with respect to the drum unit, the positional accuracy of a driving-force input portion of the development unit is not good. Therefore, an error in the meshing between the driving-force input portion of the development unit and the driving gear provided on the apparatus body may lead to nonuniformity in the transmission of the driving force and the image may be affected.
Accordingly, it is an object of the present invention to provide an image forming apparatus and a process cartridge in which the contact pressure produced between a photosensitive drum and a developing roller is controllable with high accuracy without causing a meshing error of a driving gear and image quality can be thus improved.